1. Field of the Invention
The present invention relates to a method for stabilizing an output of higher harmonic waves used in the fields of optical information processing, optical application measurement control, and the like utilizing coherent light, and a short wavelength laser beam source using the method.
2. Description of the Related Art
In the field of optical information processing, short wavelength laser beam sources for optical recording require an output of several mW or more. As blue laser beam sources, the combination of a semiconductor laser emitting fundamental waves and a light wavelength conversion device generating higher harmonic waves of the fundamental waves is promising.
FIG. 22 is a cross-sectional view showing a structure of a conventional short wavelength laser beam source 5000 emitting blue light. Fundamental waves P1 emitted by a semiconductor laser 121 are collimated by a collimator lens 124 and focused onto an optical waveguide 102 formed inside of a light wavelength conversion device 122 by a focus lens 125. The fundamental waves P1 are converted into higher harmonic waves P2 in the optical waveguide 102 and output. Each component of the short wavelength laser beam source 5000 is mounted on a base member 120 made of Al. The light wavelength conversion device 122 is positioned on a quartz plate 123 with its face having the optical waveguide 102 down.
Next, the light wavelength conversion device 122 used in the conventional short wavelength laser beam source 5000 will be described.
FIG. 23A is a perspective view of the conventional light wavelength conversion device 122; FIG. 23B is a cross-sectional view taken along a line 23Bxe2x80x9423B of FIG. 23A. Hereinafter, the operation of the light wavelength conversion device 122 will be described by illustrating the generation of higher harmonic waves (wavelength: 437 nm) from fundamental waves (wavelength: 873 nm) (see K. Yamamoto and K. Mizuuchi, xe2x80x9cBlue light generation by frequency doubling of a laser diode in a periodically-domain inverted LiTaO3 waveguidexe2x80x9d, IEEE Photonics Technology Letters, Vol. 4, No. 5, pp. 435-437, 1992).
As shown in FIGS. 23A and 23B, the light wavelength conversion device 122 includes the optical waveguide 102 formed in a LiTaO3 substrate 101. The optical waveguide 102 is provided with periodically domain-inverted layers (domain-inverted regions) 103. The mismatch in propagation constant between the fundamental waves P1 and the higher harmonic waves P2 to be generated is compensated by a periodic structure composed of the domain-inverted regions 103 and non-domain-inverted regions 104. This allows the fundamental waves P1 to be converted into the higher harmonic waves P2 at high efficiency so as to be output. The arrows in FIG. 23B represents the direction of a domain in each region.
Next, the principle of amplification of the higher harmonic waves in the light wavelength conversion device 122 will be described with reference to FIGS. 24A and 24B.
FIG. 24A schematically shows inner structures, namely, the direction of domains of a device 131 which have no domain-inverted regions and of a device 132 which has domain-inverted regions. The arrows in FIG. 24A represent the direction of a domain in each region.
In the device 131, domain-inverted regions are not formed and the directions of domains are aligned in one direction. When fundamental waves pass through the device 131, the waves are partially converted into higher harmonic waves. However, in the structure of the device 131, an output of higher harmonic waves 131a merely repeats increasing and decreasing along the passing direction of the optical waveguide, as shown in FIG. 24B.
On the other hand, in the device 132 which has first-order periodically domain-inverted regions, an output of higher harmonic waves 132a increases in proportion to the square of length L of the optical wavelength as shown in FIG. 24B. It should be noted that only when a quasi-phase match is established, the output of the higher harmonic waves P2 can be obtained from the incident fundamental waves P1 in the domain-inverted structure. The quasi-phase match is established only when a period xcex91 of the domain-inverted region is identical with xcex/(2(N2xcfx89xe2x88x92Nxcfx89)), where Nxcfx89 is an effective refractive index of the fundamental waves (wavelength: xcex), and N2xcfx89 is an effective refractive index of the higher harmonic waves (wavelength: xcex/2).
A method for producing a conventional light wavelength conversion device 5000 having the above-mentioned domain-inverted structure as a fundamental structure component will be described.
First, a periodic Ta film pattern with a width of several xcexcm is formed on the LiTaO3 substrate 101 made of non-linear optical crystal by vapor deposition and photolithography. The Ta film pattern is subjected to a proton-exchange treatment at 260xc2x0 C., followed by being heat treated at around 550xc2x0 C. Thus, the domain-inverted regions 103 are formed in the LiTaO3 substrate 101. Then, a Ta film slit is formed on the LiTaO3 substrate 101, heat treated in pyrophosphoric acid at 260xc2x0 C. for 12 minutes, and subjected to an anneal treatment at 420xc2x0 C. for one minute. Thus, the optical waveguide 102 is formed.
When the optical waveguide 102 has a length of 10 mm and the fundamental waves P1 having a power of 37 mW with respect to a wavelength of 873 nm is input to the light wavelength conversion device 122 produced as described above, higher harmonic waves P2 having a power of 1.1 mW can be output.
However, allowable width of the light wavelength conversion device 122 with respect to the wavelength of the fundamental waves is generally as small as 0.1 nm. For this reason, the light wavelength conversion device 122 cannot allow mode hopping of a semiconductor laser and spreading of the wavelength of output light.
For example, in the conventional light wavelength conversion device 122 having the above-mentioned domain-inverted regions, the allowance with respect to the wavelength fluctuation of a fundamental wave laser beam at a device length of 10 mm is very narrow; typically, an allowable wavelength half value width of around 0.1 nm. The allowable change with respect to temperature is typically as small as 3xc2x0 C. Because of this, when a light wavelength conversion device is combined with a semiconductor laser, the following problems arise: The output of the semiconductor laser is likely to be affected by the change in temperature and consequently wavelength fluctuation occurs in output light; as a result, fundamental waves are not converted into higher harmonic waves or the output of higher harmonic waves converted from fundamental waves greatly fluctuates.
The above-mentioned problems will be described in detail below.
Typically, when the wavelength of a semiconductor laser shifts by only 0.05 nm, the output of higher harmonic waves to be obtained becomes half of an intended value. The allowability with respect to the change in wavelength of a semiconductor laser is small. For example, when the ambient temperature during the operation of a semiconductor laser shifts from 20xc2x0 C. to 21xc2x0 C., the vertical mode of the semiconductor laser shifts by one and the oscillation wavelength shifts from 820.0 nm to 820.2 nm. Because of this, the output of higher harmonic waves becomes zero.
Regarding the allowable width of the light wavelength conversion device 122 with respect to the change in temperature, when the ambient temperature changes, the output of higher harmonic waves cannot be obtained even if the oscillating wavelength of the semiconductor laser is stable. Furthermore, frequent occurrence of mode hopping causes noise leading to problems in reading from optical disks.
According to one aspect of the present invention, a method for stabilizing an output of higher harmonic waves includes the steps of: converting fundamental waves, emitted from a distribution Bragg reflection (DBR) semiconductor laser having a wavelength variable portion, into higher harmonic waves in a light wavelength conversion device; and controlling a current to be applied to the wavelength variable portion of the DBR semiconductor laser to change an oscillating wavelength of the DBR semiconductor laser, thereby matching the oscillating wavelength with a peak of the higher harmonic waves.
Alternatively, a method for stabilizing an output of higher harmonic waves of the present invention includes the steps of: converting fundamental waves emitted from a semiconductor laser into higher harmonic waves in a light wavelength conversion device; applying an optical feed-back to the semiconductor laser to set an oscillating wavelength of the semiconductor laser at a predetermined value; and controlling a drive current of the semiconductor laser to change the oscillating wavelength, thereby matching the oscillating wavelength with a peak of the higher harmonic waves.
Alternatively, a method for stabilizing an output of higher harmonic waves of the present invention includes the steps of: converting fundamental waves, emitted from a DBR semiconductor laser having a first wavelength variable portion and a second wavelength variable portion, into higher harmonic waves in a light wavelength conversion device; and coarse-controlling an oscillating wavelength of the DBR semiconductor laser by the first wavelength variable portion and fine-controlling the oscillating wavelength by the second wavelength variable portion, thereby matching the oscillating wavelength with a peak of the higher harmonic waves.
Alternatively, a method for stabilizing an output of higher harmonic waves of the present invention includes the steps of: converting fundamental waves emitted from a DBR semiconductor laser having a wavelength variable portion into higher harmonic waves in a light wavelength conversion device; and performing differential detection of the output of the higher harmonic waves, controlling a current to be applied to the wavelength variable portion of the DBR semiconductor laser by using a detection result to change an oscillating wavelength of the DBR semiconductor laser, thereby matching the oscillating wavelength with a peak of the higher harmonic waves.
Alternatively, a method for stabilizing an output of higher harmonic waves of the present invention includes the steps of: converting fundamental waves emitted from a wavelength-locked semiconductor laser into higher harmonic waves in a light wavelength conversion device having an allowable wavelength half value width wider than an oscillating vertical mode interval of the semiconductor laser; and controlling a current to be applied to the semiconductor laser to change an oscillating wavelength of the semiconductor laser, thereby matching the oscillating wavelength with a peak of the higher harmonic waves.
In one embodiment of the present invention, the light wavelength conversion device is an optical waveguide type.
In another embodiment of the present invention, the optical wavelength conversion device is a bulk type.
In still another embodiment of the present invention, an output of the fundamental waves is monitored to control the current.
In still another embodiment of the present invention, a reflector is further provided between a cleavage face of the semiconductor laser and a DBR portion so that a vertical mode interval is set to be 1 nm or larger.
In still another embodiment of the present invention, the wavelength variable portion or first wavelength variable portion in the DBR semiconductor laser is positioned on a side far away from the light wavelength conversion device.
In still another embodiment of the present invention, the DBR semiconductor laser or the semiconductor laser as well as the light wavelength conversion device are mounted on a base member, an active layer of the DBR semiconductor laser and an optical waveguide of the light wavelength conversion device are respectively positioned on a side far away from the base member.
According to another aspect of the present invention, a short wavelength laser beam source includes: a light wavelength conversion device having periodically domain-inverted regions formed in non-linear optical crystal; and a DBR semiconductor laser, wherein the DBR semiconductor laser has a wavelength variable portion, fundamental waves emitted from the DBR semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, and an oscillating wavelength of the DBR semiconductor laser is changed so as to match with a peak of the higher harmonic waves by controlling a current to be applied to the wavelength variable portion of the DBR semiconductor laser, whereby a constant output of the higher harmonic waves is obtained.
Alternatively, a short wavelength laser beam source of the present invention includes: a light wavelength conversion device having periodically domain-inverted regions formed in non-linear optical crystal; and a DBR semiconductor laser, wherein fundamental waves emitted from the DBR semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device.
Alternatively, a short wavelength laser beam source of the present invention includes: a light wavelength conversion device having periodically domain-inverted regions formed in non-linear optical crystal; and a semiconductor laser, wherein fundamental waves emitted from the semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, and an oscillating wavelength of the semiconductor laser set at a predetermined value by optical feedback is changed by controlling a drive current of the semiconductor laser, thereby matching the oscillating wavelength with a peak of the higher harmonic waves to obtain a constant output of the higher harmonic waves.
Alternatively, a short wavelength laser beam source of the present invention includes: a light wavelength conversion device having periodically domain-inverted regions formed in non-linear optical crystal; and a DBR semiconductor laser having first wavelength variable portion and second wavelength variable portion, wherein fundamental waves emitted from the DBR semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, the first wavelength variable portion coarse-controls an oscillating wavelength of the DBR semiconductor laser, and the second wavelength variable portion fine-controls the oscillating wavelength, whereby the oscillating wavelength is matched with a peak of the higher harmonic waves to obtain a constant output of the higher harmonic waves.
Alternatively, a short wavelength laser beam source of the present invention includes: a DBR semiconductor laser having first wavelength variable portion; and a light wavelength conversion device having second variable portion and periodically domain-inverted regions formed in non-linear optical crystal, wherein fundamental waves emitted from the DBR semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, the first wavelength variable portion coarse-controls an oscillating wavelength of the DBR semiconductor laser, and the second wavelength variable portion fine-controls a phase-matched wavelength of the light wavelength conversion device, whereby the oscillating wavelength is matched with a peak of the higher harmonic waves to obtain a constant output of the higher harmonic waves.
Alternatively, a short wavelength laser beam source of the present invention includes: a wavelength-locked semiconductor laser; and a light wavelength conversion device having an allowable wavelength half value width wider than an oscillating vertical mode interval of the semiconductor laser, wherein fundamental waves emitted from the semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, and an oscillating wavelength of the semiconductor laser is changed by controlling a current to be applied to the semiconductor laser so as to match with a peak of the higher harmonic waves, whereby a constant output of the higher harmonic waves is obtained.
Alternatively, a short wavelength laser beam source of the present invention includes: a light wavelength conversion device having periodically domain-inverted regions formed in non-linear optical crystal; and a DBR semiconductor laser having a wavelength variable portion, wherein a reflector is provided outside the DBR semiconductor laser and a laser oscillation is generated between the reflector and the DBR semiconductor laser, fundamental waves emitted from the DBR semiconductor laser are converted into higher harmonic waves in the light wavelength conversion device, and an oscillating wavelength of the DBR semiconductor laser is changed by controlling a current to be applied to the wavelength variable portion of the semiconductor laser so as to match the oscillating wavelength with a peak of the higher harmonic waves, whereby a constant output of the higher harmonic waves is obtained.
Alternatively, a short wavelength laser beam source of the present invention includes: a light wavelength conversion device having at least three periodically domain-inverted regions formed in non-linear optical crystal; and a semiconductor laser, wherein the at least three periodically domain-inverted regions have a first periodically domain-inverted region having a period of xcex9, a second periodically domain-inverted region having a period of xcex91, and a third periodically domain-inverted region having a period of xcex92, the relationship between the periods is xcex91 less than xcex9 less than xcex92, and higher harmonic waves generated in the second periodically domain-inverted region having a period of xcex91 and higher harmonic waves generated in the third periodically domain-inverted region having a period of xcex92 are detected by different detectors, respectively.
In one embodiment of the present invention, the light wavelength conversion device is an optical waveguide type. Preferably, the optical waveguide is a proton-exchanged optical waveguide.
In another embodiment of the present invention, the light wavelength conversion device is a bulk type.
In still another embodiment of the present invention, the non-linear optical crystal is LiNbxTa1xe2x88x92xO3 (0xe2x89xa6Xxe2x89xa61).
In still another embodiment of the present invention, the above-mentioned short wavelength laser beam source further includes a detector and a beam splitter.
In still another embodiment of the present invention, an output of the fundamental waves is monitored to control the current.
In still another embodiment of the present invention, a reflector is further provided between a cleavage face of the semiconductor laser and a DBR portion so that a vertical mode interval is set to be 1 nm or larger.
In still another embodiment of the present invention, a reflector is further provided on either of an incident face or an output face of the light wavelength conversion device.
In still another embodiment of the present invention, reflected return light of the fundamental waves in the light wavelength device is 0.2% or less.
In still another embodiment of the present invention, the DBR semiconductor laser is RF-driven.
In still another embodiment of the present invention, temperature of the semiconductor laser is controlled on a first face of a Peltier device, temperature of the light wavelength conversion device is controlled on a second face of the Peltier device, and change in temperature of the first face is opposite to change in temperature of the second face.
In still another embodiment of the present invention, a wavelength of the fundamental waves is shifted from a phase-matched wavelength of the light wavelength conversion device to modulate an output of the higher harmonic waves.
In still another embodiment of the present invention, a wavelength of the fundamental waves is matched with a phase-matched wavelength of the light wavelength conversion device, and thereafter, a drive current of the semiconductor laser is regulated so as to regulate the output of the higher harmonic waves.
In still another embodiment of the present invention, the wavelength variable portion or the first wavelength variable portion in the DBR semiconductor laser is positioned on a side far away from the light wavelength conversion device.
In still another embodiment of the present invention, the DBR semiconductor laser or the semiconductor laser as well as the light wavelength conversion device are mounted on a base member, an active layer of the DBR semiconductor laser and an optical waveguide of the light wavelength conversion device are respectively positioned on a side far away from the base member.
Thus, the invention described herein makes possible the advantages of (1) providing a method for stabilizing an output of higher harmonic waves enabling the stable output of higher harmonic waves to be obtained irrespective of ambient temperature and (2) providing a short wavelength laser beam source using the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.